Background: All women have an immediate period of increased risk for developing breast cancer after childbirth, regardless of their age. Among women who have children at a relatively young age, pregnancy does eventually provide long term protection against breast cancer. However, older first-time mothers have a lifelong elevated risk for breast cancer. Currently accepted definitions of pregnancy-associated breast cancer (PABC) include cases diagnosed during pregnancy and within 1 year postpartum. However, peak incidence of PABC occurs at 5-7 years postpartum. In addition, several studies identify diagnosis up to 5 years postpartum as an independent predictor of poor prognosis, whereas diagnosis during pregnancy is not. These data identify cancers diagnosed within 5 years of a completed pregnancy as a high risk subset of PABC and provide important rationale for expansion of the definition of PABC to include later postpartum cases. Breast cancer is the leading cancer in women age 20-50, more than the 5 other top cancers combined in this age range. Parity influences the outcome of breast cancer in young women, with 34% mortality at 5 years for women within 5 years of childbirth at diagnosis, fully justifying a research focus on thi subset of breast cancer. In animal models, we demonstrate that wound healing-like programs required to remodel the lactation-competent gland to its pre-pregnant state are tumor promotional. Further, we show that NSAIDs can inhibit these tumorigenic attributes of involution. Our overall goal is to determine whether postpartum breast involution in women represents a window of opportunity for the reduction of PABC. Given that important details of postpartum human breast involution are unknown; our rodent studies do not directly address the relevance of NSAID based intervention for women. Further, we lack applicable safety data for human trials. In this proposal, we identify the missing data required to translate our PABC prevention strategy to postpartum women and propose specific aims to address each of these needs. Aim 1a- Perform a Phase 0 study in unaffected post- partum/post-lactation women to demonstrate that normal human involution is a dominant, cell death program, which occurs rapidly and completely in the majority of women after the delivery of a child whether or not she lactates. We will obtain appropriate breast tissue specimens with detailed parity and lactation history. We will examine postpartum human breast involution using state-of-the-art quantitative IHC assays on breast biopsies utilizing markers identified in our preclinical models. Aim 1b-Will confirm the presence of a pro-tumor wound healing like stromal microenvironment and identify the presence, and preliminarily the mechanisms of, a pro- inflammatory signature in breast tissue from women undergoing normal postpartum involution. Aim 2A-Expand our preclinical studies to verify efficacy of ibuprofen in an immune competent PABC model and investigate chemopreventive strategies that can be targeted to lactating women by evaluating fish oil. Aim 2B-Confirm that multiple postpartum preventative treatments do not interfere with fertility, abilit to nurse, or involution in subsequent pregnancies. Aim 3A-Assess for tumor suppressive effects of systemic NSAID treatment on the microenvironments of common sites of breast cancer metastasis such as lung and liver. Aim 3B-Address potential interactions with autoimmune disease by determining if systemic NSAID treatment during involution alters release of self-antigen. Significance: The goal is to provide data required to propose a large scale prevention trial where women at high risk for breast cancer would take a 'postnatal' pill during postpartum breast involution to reduce the tumor promotional attributes of this unique risk window, an important objective given the 6 million U.S. pregnancies per year.